Idiopathic thrombocytopenic purpura (ITP), heparin-induced thrombocytopenia (HIT), and thrombotic thrombocytopenic purpura (TTP) are three of the most common antigen-restricted autoantibody-mediated human immunohematological disorders. Yet, the mechanism of autoantibody-induced thrombocytopenia and the natural histories of these disorders differ in ways that suggest fundamental differences in the evolution of autoantibody formation. Children with ITP have a self-limited disease suggesting incitement by an evanescent exogenous antigen with subsequent re-establishment of normal tolerance mechanisms, whereas autoantibody formation persists throughout life in most adults with this disease. Studies of HIT demonstrate that essentially all immunologically "healthy" individuals treated with heparin generate anti-PF4/heparin antibodies suggesting either an anamnestic response to endogenous antigen or polyclonal stimulation by PF4/heparin complexes, e.g. as a superantigen. TTP offers yet a third, ill-studied paradigm in which the clinical disease is transient in 85% of patients, suggesting either a naturally self-limited autoantibody production to ADAMTS13 or a transient enhancement of vWF oligomerization due to vascular injury in the face of persistence of potentially pathogenic antibody. We hypothesize that the fundamental differences in autoantibody production in ITP, HIT and TTP can be explored by determining the genetic restriction and pattern of somatic mutation of the autoantibody repertoire, the role autoantigen plays in the development of the repertoire, the clonality in B-cell response, and the identity of pathogenic B-cell epitopes. This information would provide insight into the molecular basis of pathogenicity and lead to the development of specific and more effective therapies. We propose to utilize several antibody and peptide phage display-based technologies developed in our laboratory to rapidly clone and characterize human monoclonal autoantibodies and test this hypothesis through three interrelated specific aims focused on the autoimmune response in each of these disorders. A comprehensive understanding of the genetic and immunological properties of disease-associated autoantibodies will comprise the essential first step for the creation of novel approaches for antigen-specific modulation such as tolerance induction, peptide-based inhibitors, and DNA vaccination that induces regulatory T-cell responses. Epitope mapping of these antibodies and isolation of peptides based on the antigen-recognition sites will provide valuable reagents for structure/function analysis of the respective autoantigens and the means needed to rationally and specifically modulate their functions in vitro and in vivo.